Microelectronic devices, such as semiconductor devices, imagers, and displays, are generally fabricated on and/or in microelectronic workpieces using several different types of machines. In a typical fabrication process, one or more layers of conductive materials are formed on a workpiece during deposition steps. The workpieces are then typically subject to etching and/or polishing procedures (e.g., planarization) to remove a portion of the deposited conductive layers, to form contacts and/or conductive lines.
Electroplating processors can be used to deposit copper, solder, permalloy, gold, silver, platinum, electrophoretic resist and other materials onto workpieces for forming blanket layers or patterned layers. A typical copper plating process involves depositing a copper seed layer onto the surface of the workpiece using chemical vapor deposition (CVD), physical vapor deposition (PVD), electroless plating processes, or other suitable methods. After forming the seed layer, a blanket layer or patterned layer of copper is plated onto the workpiece by applying an appropriate electrical potential between the seed layer and one or more electrodes in the presence of an electroprocessing solution. The workpiece is then cleaned, etched and/or annealed in subsequent procedures before transferring the workpiece to another processing machine.
As microelectronic features and components are made ever smaller, the thickness of the of the seed layer deposited into or onto them must also be made ever smaller. Electroplating onto thin seed layers presents substantial engineering challenges due to the terminal effect. The terminal effect results due to a large voltage drop across the wafer diameter, caused by the high resistance of the seed layer. If not adequately compensated, the terminal effect causes the electroplated layer to be non-uniform, and it may also cause voids within the features. With very thin seed layers, the sheet resistance at the start of the electroplating process may be as high as, for example 50 Ohm/sq, whereas the final sheet resistance of the electroplated film on the workpiece may be below 0.02 Ohm/sq. With conventional electroplating tools, this three orders of magnitude change in sheet resistance can make it difficult or impossible to consistently provide uniform void-free films on workpieces. Accordingly, improved electroplating tools are needed.